This is an extremely difficult issue to tackle: how can we study something that has not happened? One way to approach it is to start with a question used by those who deny evolution and believe that many of nature’s inventions, such as the eye or the bacterial flagellum, are simply too complex to have evolved. What use is half a wing, they ask? (see Half a wing is no use)

Very useful, it turns out. The wings of insects might have evolved from flapping gills that were originally used for rowing on the surface of water. This is an example of exaptation – structures and behaviours that evolved for one purpose but take on a wholly new one, while remaining useful at every intermediate stage.

Come in, over

Turn this argument around, however, and it suggests that some features cannot evolve because a half-way stage really would be of no use. For example, two-way radio might be useful for many different animals, for making silent alarm calls or locating other members of your species. So why hasn’t it evolved? The recent invention of nanoscale radio receivers suggests it is not physically impossible.

The answer might be that half a radio really is useless. Detecting natural radio waves – from lightning, for instance – would not tell animals anything useful about their environment. That means there will be no selection for mutations that allow organisms to detect radio waves. Conversely, without any means of detecting radio waves, emitting them would serve no useful purpose. Radar might not be able to evolve for similar reasons.

The contrast with visible light could hardly be greater. It is clear that simplydetecting the presence or absence of light would be advantageous in many environments, that even a blurry picture is better than nothing at all, and so onright up to hawk-eyed sharpness.

Seaweed skies

Emitting visible light can be helpful too, even for creatures that cannot detect it themselves. For the bioluminescent phytoplankton that light up ocean waves, for instance, it is a way of summoning predators that eat the phytoplankton’s enemies. A similar argument applies to sound: it is not hard to see how forms of echolocation evolved independently in groups such asbats, cave swiftlets and whales.

One might also wonder why plants that float in the sky like balloons have never evolved. The idea does not seem too far-fetched at first glance: many seaweeds have floats called pneumatocysts, filled with oxygen or carbon dioxide. Other algae can produce hydrogen. So fill a large, thin pneumatocyst with hydrogen and perhaps a seaweed could fly. Flying plants would beat water and land plants to the light, giving them a big advantage, so why aren’t our skies filled with living green balloons?

Perhaps partly because large pneumatocysts with extremely thin membranes would be far more vulnerable to predators and damage from waves, so an intermediate stage could never evolve. What’s more, algae produce hydrogenonly when there’s a lack of sulphur in the water, and in any case the molecules of hydrogen gas are so tiny that they would leak out of any pneumatocyst. Half a hydrogen balloon doesn’t look very good for anything, at least on our planet. Even evolution has its limits.